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Conclusion

Set in a NNW verging thrust sheet, the outcrop appears to be near the base of a thrust fault, possibly a splay of the Cooper Creek Reservoir fault. The timing of the folding is assumed coincident with the shortening that also produced offset on the Bonanza and Cooper Creek Reservoir faults. This places an upper constraint of 52 Ma, end of the early Eocene, before deposition of the Camas Valley Formation in earliest middle Eocene, a period of transition to the north-south structural alignment of the Tyee basin. A constraint on the onset of folding is lithification of the early Eocene beds.

The exposure of folded turbidite beds displays a complex interplay of folding and faulting. Medial axis shortening attributable to folding is a minimum of 35 to 40 percent, with additional shortening caused by bed repetition in faulting and likely bed thickening. Thrust faults consistent with layer-parallel shortening dominate the system of fractures in the outcrop, occurring before, during, and after buckling of layers to create folds.

In the order of deformation, localized boudins formed before or early in the layer-parallel shortening (Figure 39). Conjugate faulting or weakening, and duplication of beds by ramp faulting, also appear to have initiated early in the shortening, possibly creating a nucleus for folding. The folds of Sections II to VI were subject to a principal compressive stress oblique to layering, creating a mean top-to-the-south layer-parallel shear, with chevron-like folds facing south and a kink-like fold facing north. Post folding horizontal shortening is evident in the rotation of limbs and in a conjugate faulting and jointing pattern in the outcrop, especially in vertical beds.

Figure 39. Order of deformation. A. Formation of boudins, and B. initiation of conjugate faults appear early in the deformation. C. Bed duplication with ramp faults also initiates early in the shortening, continuing as folding proceeds. D. Folding on different scales continues subject to a net top-to-the-south layer-parallel shear. E. Wedges form during folding. F. Sub-horizontal shortening continues with conjugate faults in steep beds and rotation of limbs.

Conditions in the multilayer that contributed to the final fold form are

1. A relatively thick stiff unit above the thin beds in the upper part of the concentric-like fold contributed to the concentric-like form similar to the upper layers in a constrained multilayer.
2. A mean top-to-the-south sense of layer-parallel shear produced an opposite sense of asymmetry in the kink-like fold compared with the chevron-like folds. The short limb of the kink-like fold faces north and the chevron-like folds face south. The top-to-the-south sense of layer-parallel shear is consistent with the setting in a thrust sheet with NNW vergence and overshadows possible top-to-the-north layer-parallel shear caused by flexural slip on the south limb of the Oakland anticline.
3. The localized tan siltstone and associated calcareous nodules amalgamated to the sandstone layers within the kink-like fold may be evidence of nonlinear slip on the layer contacts which theoretically is necessary for the formation of ideal monoclinal kink folds.
4. The wavelength of the folds in the outcrop varies according to the thickness of the folding units.
5. Wavelength to thickness models yield initial thickness of 8 m in Section I and 15 m in Section VI, corresponding to layer to average media viscosity contrasts of 0.5 and 0.8 respectively.
6. Modeling the number of structural units in Section I and VI multilayers resulted in two units each, perhaps an indication the layer contacts were more bonded than free slip.
7. Boudins in the calcareous layers throughout the outcrop are evidence of non-linear material behavior. Coincident with the calcareous layers and boudins is thickening of adjacent layers or more conspicuous jointing.

The exposure of folds is a cross-section of folding units on the southeast limb of the Oakland anticline going down-section from south to north. This cross-section reveals only short wave-trains going from one folding unit to the next. The apparent folding units going down-section, with wavelengths and total bed thickness:

The total measured thickness of the outcrop is about 100 m, with estimated overburden at the time of folding of about 1300 m. The relative thinness of the turbidite unit compared with the Tenmile Formation as a whole, and its strength contrast with the muddier units above and below, allowed the folds to form in layer-parallel shortening at the outcrop wavelengths.

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